How to make every single part of a complex geometry thinner?
Here are two ways you might be able to go about doing this, though they aren't going to provide you perfect results. Both of these techniques involve moving all faces in along their normals.
Option 1
In edit mode, select everything and hit ALT+S, then slowly draw your mouse down to shrink. You may wish to select the Offset Even checkbox in the little settings box.
Option 2
Add a displace modifier to your mesh. Leave the Midlevel at 0.5, and set the strength to 0.0. Then, slowly draw the strength into the negative numbers, which will shrink the mesh. The exact number will depend heavily on the scale of your mesh, and the degree to which you need it thinned.